The present invention relates to a laser annealing method and laser annealing apparatus, which are capable of providing an improvement in the film qualities, the crystal grain enlargement, or single-crystallization of an amorphous or polycrystalline semiconductor film that is formed on an insulating substrate through laser beam irradiation; and, more particularly, the invention relates to a TFT device that is manufactured through laser annealing and to a display apparatus provided with such a TFT device.
Currently, images are produced on liquid crystal panels by switching thin film pixel transistors that are formed with an amorphous or polycrystalline silicon film on a substrate of glass or fused quartz. Significant reductions in production costs and improvements in reliability could be expected if it were possible to form a driver circuit, for driving the pixel transistor, on the substrate at the same time.
However, because the silicon film forming the active layer of the transistors has a poor crystalline, the capacity of thin film transistors, as represented by the mobility, is low, and the manufacture of circuits for which high speeds and high-performance are required is difficult. A high mobility thin film transistor is required in order to manufacture these high-speed, high performance circuits; and, in order to realize this, improvements in the crystallinity of the silicon thin film are necessary.
Excimer laser annealing has, in the past, attracted attention as a method for improving the crystallinity. In this method, the mobility is improved by changing the amorphous silicon film to a polycrystalline silicon film, and this is accomplished by irradiating an excimer laser on an amorphous silicon film, that is formed on an insulating substrate of glass or the like. However, the polycrystalline film attained by excimer laser irradiation has crystal grain sizes of only several hundred nanometers.
Thus, the film does not have a sufficient performance to be used as part of a driver circuit or the like for driving a liquid crystal panel.
In order to resolve this issue, Japanese Patent Laid-open Application No. 2001-44120 discloses a method wherein the crystal grain sizes are increased by irradiation with an ultraviolet pulsed laser, such as an excimer laser, for example, which serves as a first pulsed laser, followed by irradiation with a pulsed laser in the visible light range, which serves as a second pulsed laser. This method is designed to increase the crystal grain sizes by extending the re-crystallization time with the radiation of the second pulsed laser within 200 nanoseconds of irradiation with the first pulse laser.